There is, as a conventional vehicle surroundings monitoring apparatus, a system in which a camera is mounted on a vehicle, and the distance to an obstacle presenting a risk of collision is detected from a captured image of the vehicle's surroundings. When a vehicle-mounted camera is used, in particular, it is important to be able to perform stable detection even in the event of changes in the camera's attitude, such as pitch (vertical rocking motion) due to the behavior of the vehicle.
Thus, a method is known whereby an attitude change amount is calculated by means of image processing using a road boundary line or vanishing point from an image captured by a camera, and is corrected (see Patent Literature 1 and 2).
Also, a method is known whereby a plurality of objects are extracted from an image obtained by a camera, an average value of displacement amounts of height-direction positions of these objects is calculated as pitch, and coordinates are corrected (see Patent Literature 3).